3, A A A 



70 On Single and Double Vision, and an Optical Illusion. 



1. A A Natural single vision. 



2. A A A A View with axes slightly converged. 



View with greater convergence of the op- 

 tical axes and the two intermediate images 

 coalesced into one. 



On suddenly closing either eye, this middle or superimposed 

 irna°;e did not disappear, and it was evidently made of two ima- 

 ges from two objects formed on corresponding parts of the retinae. 

 Hence we have the converse of the case of double vision of a 

 single object : for two objects are made to produce a single im- 

 pression. Thus far I had proceeded in 1816, when I read a 

 paper on this subject to a club of my fellow students at Yale. 



Experiment III. — In 1843, I made the experiment of con- 

 verging the optical axes upon two contiguous figures on the 

 wall paper of my office, in the same manner as I had done with 

 the two images of the two candles in Exp. II. When I had thus 

 succeeded in taking up optically the two figures and superimpos- 

 ing them one upon the other, suddenly the whole wall appeared 

 to leap out from a distance of ten feet to within half a yard of 

 my eyes, where it remained in miniature beauty as palpable to 

 vision as it had been in its original place. To this image, sus- 

 pended as it were between the observer and the object, I shall, in 

 the subsequent part of my paper, apply the term illusive image. 



It then appeared that the right eye was directed to the left one of 

 two contiguous figures, and the left eye to the right figure, which 

 being identical in form and size, gave the impression of a single 

 object at the point of intersection of the optical axes. Here we 

 have two triangles formed by the two optical axes intersecting 

 each other and joined at their extremes, on one part by a line 

 from one eye to the other, and on the other part by a line from 

 one figure or object to the other. These last lines being parallel, 



(see figure,) where A and B represent the eyes, C and 

 D, the objects, or two figures on the wall ; AD the 

 axis of the eye A, BC the axis of the eye B, and E 

 the point of intersection of the axes at the place of 

 the illusive image. As these triangles are equian- 

 gular and similar, we can deduce from them all 

 of the equations of such triangles and apply them 

 to the optical phenomena. Thus the distance from 

 the eye to the illusive image (AE) will be to the 

 distance from the object to the same image (DE) 

 as the distance between the eyes ( AB) is to the dis- 

 tance between the objects (CD) the figures or pannels on the 

 paper, &e. &c. 



It is not merely the two objects directly in the axes of the eyes 

 which coincide, but every contiguous pair of objects seen ob- 



